Repair documentation generator and broaden bundle compatibility

[Shaptic]

See related stellar/js-stellar-sdk#833.

Docs weren't working before, but they should now:

• Babel will build a modern JS version of the library for docs (if you yarn build:docs)
• jsdoc will parse that & build the docs website (if you yarn docs)
• but in production, an ES5-compatible module will get build (when you yarn prepare, etc.)

Closes #85.